NEDERBRAGT, A.J.*; LESPINET, O.; DICTUS, W.J.A.G.; VANLOON, A.E.; ADOUTTE, A.; VANDENBIGGELAAR, J.A.M.: Twist and Snail Homologues in the Gastropod Mollusk Patella Vulgata: A New Twist for an Old Gene Network?

Mesoderm development must have been crucial in the evolution from diploblastic into triploblastic animals. To obtain a reliable insight into the evolution of the mesoderm we urgently need information about the molecular control of mesoderm in other phyla. Our group is interested in the molecular aspects of mesoderm formation in Spiralia. As the twist-snail gene network is known to specify mesoderm in a number of animals, it is interesting to know whether its function in Spiralia such as Molluscs and Annelids is conserved. As a starting point we chose a primitive gastropod mollusk, Patella vulgata. We have obtained two Patella orthologues of snail, and a fragment of the orthologue of twist. The results of in situ hybridization experiments for snail are very intriguing, as no distinct mesodermal expression for either of the snail orthologues has been observed. The pattern we see indicates expression inside the future head region, in the developing mantle folds, and possibly in the developing central nervous system. In situ hybridizations for twist thus far showed several phases of expression inside the embryo, none of which clearly indicate a role in mesoderm development. The role of twist in Patella therefore remains unclear. We conclude that in molluscs, and possibly in all Spiralia, the twist-snail gene network does not function in mesoderm formation, and that the ancestral role of the snail gene family involves epithelial-mesenchymal transitions and nervous system development.